Light-emitting device and manufacturing method thereof

ABSTRACT

A light-emitting device including a substrate; a first conductivity semiconductor layer on the substrate; a first barrier on the first conductivity semiconductor layer; a well on the first barrier and including a first region having a first energy gap and a second region having a second energy gap and closer to the semiconductor layer than the first region; a second barrier on the well; and a second conductivity semiconductor layer on the second barrier; wherein the first energy gap decreases along a stacking direction of the light-emitting device and has a first gradient, the second energy gap increases along the stacking direction and has a second gradient, and an absolute value of the first gradient is smaller than an absolute value of the second gradient.

REFERENCE TO RELATED APPLICATION

This present application is a continuation application of U.S. patentapplication Ser. No. 14/636,939 filed on Mar. 3, 2015, entitled as“LIGHT-EMITTING DEVICE AND MANUFACTURING METODE THEREOF”, which claimspriority to Taiwan Application Serial No. 103130523, filed on Sep. 3,2014, and the content of which is hereby incorporated by reference inits entirety.

TECHNICAL FIELD REFERENCE TO RELATED APPLICATION

The disclosure is related to a light-emitting device, and moreparticularly, a light-emitting device with a quantum well structure.

DESCRIPTION OF THE RELATED ART

In comparison with conventional light sources, the light-emitting diodewith longer service life, smaller volume, lighter weight, and higherefficiency is widely adopted in optical display devices, traffic lights,information storage apparatuses, communication apparatuses, lightingapparatuses, and medical appliances. A light-emitting diode can be usedsolely or connected to other devices for forming a light-emittingdevice. For example, a light-emitting diode can be disposed on asubstrate and then connected to a side of a carrier or soldered/glued ona carrier for forming a light-emitting device. Additionally, the carrierfurther includes an electrode which is electrically connected to thelight-emitting device.

Generally, a light-emitting diode may include an n-type semiconductorlayer, an active layer, and a p-type semiconductor layer. In order toenhance the light efficiency of light-emitting devices, a multi-quantumwell structure is formed in the active layer. How to enhance the lightefficiency by a quantum well structure becomes a major topic forimproving the performance of light-emitting diodes.

SUMMARY OF THE DISCLOSURE

The disclosure is relative to a light-emitting device including asubstrate; a first conductivity semiconductor layer on the substrate; afirst barrier on the first conductivity semiconductor layer; a well onthe first barrier and including a first region having a first energy gapand a second region having a second energy gap and closer to thesemiconductor layer than the first region; a second barrier on the well;and a second conductivity semiconductor layer on the second barrier;wherein the first energy gap decreases along a stacking direction of thelight-emitting device and has a first gradient, the second energy gapincreases along the stacking direction and has a second gradient, and anabsolute value of the first gradient is smaller than an absolute valueof the second gradient.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing is included to provide easy understanding ofthe present application, and is incorporated herein and constitutes apart of this specification. The drawing illustrates the embodiment ofthe present application and, together with the description, serves toillustrate the principles of the present application.

FIG. 1A shows a cross section of a light-emitting device in accordancewith a first embodiment of the present application.

FIG. 1B shows a detailed view of FIG. 1A.

FIG. 1C show a detailed alignment view of FIG. 1B.

FIG. 2A shows flow rates as functions of time while forming a well and abarrier in accordance with the first embodiment of the presentapplication.

FIG. 2B shows a diagram of the well and the barrier in accordance withthe first embodiment of the present application.

FIG. 2C shows the operational temperature as a function of time whileforming the well and the barrier in accordance with the first embodimentof the present application.

FIG. 2D shows energy bands and structures of the well and the barrier inaccordance with the first embodiment of the present application.

FIG. 3A shows flow rates as functions of time while forming a well and abarrier in accordance with a second embodiment of the presentapplication.

FIG. 3B shows a diagram of the well and the barrier in accordance withthe second embodiment of the present application.

FIG. 3C shows the operational temperature as a function of time whileforming the well and the barrier in accordance with the secondembodiment of the present application.

FIG. 3D shows energy bands and structures of the well and the barrier inaccordance with the second embodiment of the present application.

FIG. 4 shows energy bands of the wells and the barriers oflight-emitting devices in accordance with the first embodiment and thesecond embodiment of the present application and the conventional art.

FIG. 5 shows internal quantum efficiency as functions of power for thelight-emitting devices in accordance with the first embodiment and thesecond embodiment of the present application and the conventional art.

FIG. 6A shows output power as functions of current density for thelight-emitting devices in accordance with the first embodiment and thesecond embodiment of the present application and the conventional art.

FIG. 6B shows normalized efficiency as functions of current density forthe light-emitting devices in accordance with the first embodiment andthe second embodiment of the present application and the conventionalart.

FIG. 7A shows concentration of carriers as functions of position andenergy band as functions of position for the well and the barrier forthe light-emitting device in accordance with the conventional art.

FIG. 7B shows energy bands of the well and the barrier and Femi energiesof electrons and holes for the light-emitting device in accordance withthe conventional art.

FIG. 8A shows concentration of carriers as functions of position andenergy band as functions of position for the well and the barrier inaccordance with the first embodiment of the present application.

FIG. 8B shows energy bands of the well and the barrier and Femi energiesof electrons and holes in accordance with the first embodiment of thepresent application.

FIG. 9A shows concentration of carriers as functions of position andenergy band as functions of position for the well and the barrier inaccordance with the second embodiment of the present application.

FIG. 9B shows energy bands of the well and the barrier and Femi energiesof electrons and holes in accordance with the second embodiment of thepresent application.

FIG. 10 shows a simulation of the recombination rate as functions ofposition for the light-emitting devices in accordance with the firstembodiment and the second embodiment of the present application and theconventional art.

FIG. 11 shows a simulation of the normalized efficiency as functions ofcurrent density for the light-emitting devices in accordance with thefirst embodiment of the present application and the conventional art.

DETAILED DESCRIPTION OF THE EMBODIMENTS

To better and concisely explain the present application, the same nameor the same reference number given or appeared in different paragraphsor figures along the specification should has the same or equivalentmeanings while it is once defined anywhere of the present application.

The following shows the description of embodiments of the presentapplication in accordance with the drawing.

FIG. 1A shows a cross section of a light-emitting device in accordancewith an embodiment of the present application. A light-emitting device 1includes a substrate 10, a nucleation layer 20, a buffer layer 30, afirst conductivity semiconductor layer 40, a strain releasing stack 50,an active layer 60, a second conductivity semiconductor layer 70, afirst electrode 80, and a second electrode 90. In the embodiment, theabovementioned layers are epitaxially grown on the substrate 10 byapproaches such as metal organic chemical vapor deposition (MOCVD) ormolecular-beam epitaxy (MBE) and the growth direction is indicated by anarrow C_(N) The substrate can be a single crystal substrate, anelectrically conductive substrate, or an insulating substrate. Theelectrically conductive substrate can be a silicon substrate, a galliumnitride substrate, or a silicon carbide substrate. The insulatingsubstrate can be a sapphire substrate. In the embodiment, each layer isepitaxially grown on the C plane of the sapphire substrate by MOCVD andthe substrate 10 optionally has a patterned surface by etching forenhancing light extraction efficiency. Additionally, for epitaxiallygrowing the layers, trimethylgallium (TMGa), triethylgallium (TEGa),trimethylaluminum (TMAl), and trimethylindium (TMIn) can be used asgroup IIIA sources; ammonia (NH₃) can be used as a group VA source;silane (SiH₄), and bis-cyclopentadienyl magnesium (Cp₂Mg) can be used asdopant sources.

In order to reduce lattice mismatch between the substrate 10 and thefirst conductivity semiconductor layer 40, the nucleation layer 20 andthe buffer layer 30 can be sequentially formed between the substrate 10and the first conductivity semiconductor layer 40. The thicknesses ofthe nucleation layer 20 and the buffer layer 30 can be tens ofnanometers (for example, 30 nm) and several micrometers, (for example, 3μm) respectively. Materials of the nucleation layer and buffer layer canbe group IIIA-VA materials including but not limited to gallium nitrideor aluminum nitride.

The first conductivity semiconductor layer 40, for example, an n-typesemiconductor layer, is formed on the substrate 10, the nucleation layer20, and the buffer layer 30. In the embodiment, the thickness of thefirst conductivity semiconductor layer is several micrometers (forexample, 2.5 μm) and the material of the first conductivitysemiconductor layer 40 can be gallium nitride. A ratio of group VAsource (for example, ammonia) to the group IIIA source (for example,trimethylgallium) can be 1000 for forming the first conductivitysemiconductor layer 40. Additionally, by introducing silane as a dopingsource, GaN layer with silicon dopants can be formed and functions asthe first conductivity semiconductor layer 40. The material of the firstconductivity semiconductor layer 40 is not limited hereto and can beother group IIIA-VA material.

Similarly, for reducing the lattice mismatch between the firstconductivity semiconductor layer 40 and the active layer 60 to decreasethe crystal defects, a strain releasing stack 50 can be formed on thefirst conductivity semiconductor layer 40. The strain releasing stack 50can have a superlattice structure by alternately stacking two kinds ofsemiconductor layers with different materials. The two kinds ofsemiconductor layers can be an indium gallium nitride layer (InGaN) anda gallium nitride layer (GaN), and thicknesses of the indium galliumnitride layer and the gallium nitride layer can be hundreds ofnanometers (for example, 120 nm). Otherwise, the strain releasing stack50 can be multi-layers with different materials and similar efficacy.

The active layer 60 is formed after the strain releasing stack 50 isformed. Please refer to FIGS. 1B and 1C. FIG. 1B shows a detailed viewof FIG. 1A and FIG. 1C show a detailed alignment view of FIG. 1B. In theembodiment, the active layer 60 includes a multi-quantum well structurebut is not limited to it. In other embodiment, the active layer caninclude a single quantum well structure and is formed by alternatelystacking a plurality of wells 601 and barriers 603. In the embodiment,one barrier 603 is firstly formed on the strain releasing stack 50, onewell 601 is formed on such barrier 603, and another barrier 603 andanother well 601 are alternately formed on such well 601 repeatedlywhile the last one is barrier 603 or well 601. The steps of forming theabovementioned multi-quantum well structure of the active layer 60 caninclude forming the well 601 first, then forming the barrier 603, andalternately forming the well 601 and the barrier 603 repeatedly. Thethickness of each of the wells 601 is several nanometers (for example, 2nm˜3 nm), and the well 601 can include three regions designated asregion I 6010, region II 6012, and region III 6014. In the embodiment,the region I 6010 is closer to the first conductivity semiconductorlayer 40 and the strain releasing stack 50. The region II 6012 isdisposed between the region I 6010 and region III 6014, and the regionIII 6014 is away from the first conductivity semiconductor layer 40 andthe strain releasing stack 50. The material of the barrier 603 caninclude a group IIIA-VA material, for example, gallium nitride oraluminum nitride. The material of the well 601 can include a groupIIIA-VA material, for example, In_(x)Ga_((1-x))N, Al_(x)Ga_((1-x))N,Al_(x)In_(y)Ga_((1-x-y))N, Al_(x)In_(1-x)N or combinations thereof,wherein 0≤x, y<1. In the embodiment, the material of the barrier 603 isgallium nitride and the material of the well 601 is gallium indiumnitride. A ratio of the group VA source (for example, ammonia) to thegroup IIIA source (for example, trimethylindium) can be 18000 forforming the well 601 of the active layer 60; a ratio of the group VAsource (for example, ammonia) to the group IIIA source (for example,triethylgallium) can be 2000 for forming the barrier 603 of the activelayer 60, but the present application is not limited hereto.

The second conductivity semiconductor layer 70 is formed on the activelayer 60. In the embodiment, the second conductivity semiconductor layer70 can be a p-type conductivity semiconductor layer, for example, agallium nitride layer doped with magnesium dopants. but the presentapplication is not limited hereto. The material of the secondconductivity semiconductor layer 70 can be other group IIIA-VA material.In the embodiment, a ratio of the group VA source (for example, ammonia)to the group IIIA source (for example, trimethylgallium) can be 5000 forforming the second conductivity semiconductor layer 70 andbis-cyclopentadienyl magnesium can be used as a magnesium dopant source.After the second conductivity semiconductor layer 70 is formed, thefirst electrode 80 and the second electrode 90 are manufactured byprocesses, such as lithography, etching, and metal deposition forcompleting the light-emitting device 1. The abovementioned firstconductivity semiconductor layer 40 and the second conductivitysemiconductor layer 70 can be single layer or multilayers. Additionally,an undoped semiconductor layer can be disposed on the first conductivitysemiconductor layer 40 or the second conductivity semiconductor layer70.

Referring to FIGS. 2A to 2D for further understanding the formation ofthe active layer 601. FIG. 2A shows flow rates as functions of timewhile forming the well 601 and the barrier 603 in accordance with thefirst embodiment of the present application. FIG. 2B shows a diagram ofthe well 601 and the barrier 603 in accordance with the first embodimentof the present application. FIG. 2C shows operational temperature as afunction of time while forming the well 601 and the barrier 603 inaccordance with the first embodiment of the present application. FIG. 2Dshows energy bands and structures of the well 601 and the barrier 603 inaccordance with the first embodiment of the present application. Asabove mentioned, the active layer 60 is formed by alternately stackingthe plurality of wells 601 and barriers 603. As shown in FIGS. 2A to 2D,the well 601 is between two barriers 603. While forming the barrier 603,a gallium based gas such as triethylgallium (TEGa), an indium based gassuch as trimethylindium (TMIn) and a nitrogen based gas such as ammonia(NH₃) are introduced. In the present embodiment, a flow rate of thegallium based gas FR1, a flow rate of the indium based gas FR2, and aflow rate of the nitrogen based gas FR3 are constants. The operationaltemperature for forming the barrier 603 can maintain at a firstpredetermined value T₁, for example, 870 degrees Celsius. A thickness ofthe barrier 603 is several nanometers to tens of nanometers (e.g., 12nm).

While forming the well 601, in an interval between t₁ and t₂ (about 160seconds), the flow rate of the gallium based gas FR1, the flow rate ofthe indium gas FR2, and the flow rate of nitrogen based gas FR3 can bemaintained at fixed values, respectively, and the operationaltemperature is decreased from the first predetermined value T₁ (e.g.,870 degrees Celsius) to a second predetermined value T₂, (e.g., 755degrees Celsius) for forming the region I 6010 of the well 601. Theoperational temperature can be decreased linearly, stepwise or in otherways. Generally speaking, while epitaxially growing layers by MOCVD, theindium content of the layer is increased as the operational temperatureis decreased. In other words, the indium content of the layer isdecreased as the operational temperature is increased. By theabovementioned approaches which adjust the operational temperature, theindium content of the region I 6010 is modulated to be increased alongthe stacking direction (indicated by arrow C_(N)) of the light-emittingdevice 1. A composition of the region I 6010 can range from GaN to In₀₂₅Ga₀ ₇₅N, but the present application is not limited hereto. The riseof the indium content of the region I 6010 can be linear, stepwise or inother ways.

After the region I 6010 is formed, the region II 6012 of the well 601 isformed in an interval between t₂ and t₃ (about 60 seconds) and theoperational temperature is maintained at the second predetermined valueT₂. Additionally, the flow rate of the gallium based gas FR1, the flowrate of the indium based gas FR2, and the flow rate of the nitrogenbased gas FR3 are maintained at predetermined values. In the intervalbetween t₂ and t₃, the operational temperature is maintained at thesecond predetermined value T₂ and thus the indium content of the regionII 6012 is substantially constant (a composition of the region III canbe maintained at In₀ ₂₅Ga₀ ₇₅N).

After the region I 6010 and the region 6012 are formed, the region III6014 of the well 601 is formed in an interval between t₃ and t₄. In theinterval between t₃ and t₄, the flow rate of the gallium based gas FR1,the indium based gas FR2, the nitrogen based gas FR3 are maintained atthe abovementioned values, and the operational temperature is increasedlinearly/stepwise to a third predetermined value T₃ so that the indiumcontent of the region III 6014 is decreased along the stackingdirection. The indium content of the region III 6014 can be decreasedlinearly or stepwise. Additionally, for the individual well 601, theregion III 6014 is closer to the second conductivity semiconductor layer70 than the region I 6010, and the region II 6012 is formed between theregion I 6010 and the region III 6014. In other words, the region I6010, the region II 6012, and the region III 6014 are formed insequence. In other embodiment, the sequence can be changed.

After the well 601 is formed, another barrier 603 is formed thereon thegallium based gas such as TEGa, the indium based gas such as TMIn, andthe nitrogen based gas such as NH₃ are introduced at the same flow rateused in forming the above-mentioned well 601 and the barrier 603, andthe operational temperature is maintained at the third predeterminedvalue T₃.

Additionally, the region I 6010 has an energy gap EI (not shown infigures). The energy gap EI is decreased linearly, stepwise or in otherways along the stacking direction (indicted by the arrow C_(N)) of thelight-emitting device 1 and has a first gradient. In the embodiment, thefirst gradient ΔEI/ΔDIis defined as an energy gap difference per unitthickness in the region I 6010 while the thickness is defined along thestacking direction C_(N). In the embodiment, indium gallium nitride(In_(x)Ga_(1-x)N) functions as the well. The energy gap EI is decreased(x becomes bigger) since the operational temperature is decreased alongthe stacking direction while forming the region I 6010 so that theindium content is increased.

In other respect, the region II 6012 has an energy gap EII (not shown infigures) Since the operational temperature is maintained at the secondpredetermined value T₂ while forming the region II 6012, the indiumcontent of the region II 6012 is substantially fixed and the energy gapEII can be regarded as a constant. In other words, the energy gap EII isdevoid of gradient variation along the stacking direction.

The region III 6014 has an energy gap EIII (not shown in figures) whichcan be increased linearly, stepwise or in other ways along the stackingdirection as above mentioned and has a second gradient ΔEIII/ΔDIIIdefined as an energy gap difference per unit thickness in the region III6014. As shown in FIG. 2D, the energy gap EI difference is smaller thanthe energy gap EIII difference from magnitude point of view. In otherwords, an absolute value of the first gradient |ΔEI/ΔDI| is smaller thanan absolute value of the second gradient |ΔEIII/ΔDIII|. It is becausewhile forming the region I 6010, the operational temperature is variedfrom the first predetermined value T₁ to the second predetermined T₂ ina longer interval between t₁ and t₂, for example, 160 seconds, so thatthe indium content in region I 6010 correspondingly varies from a lowerfraction to a higher fraction in such longer interval and theoperational temperature varies from the second predetermined value T₂ tothe third predetermined value T₃ in a shorter interval between t₃ andt₄, for example, 60 seconds, so that the indium content in region III6014 varies from a higher fraction to a lower fraction in such shortinterval. Additionally, as shown in FIG. 2D, an average of the energygap EI and an average of the energy gap EIII are greater than the energygap EII and the energy gap of the barrier 603 is greater than the energygap of the well 601.

In the embodiment, although the indium content of each region isadjusted by the operational temperature so that the energy gaps ofdifferent regions of the well are varied, the present application is notlimited to adjust the operational temperature or the aforementionedgases, and the adjusted content is not limited to indium content. Inother embodiment, metal content, for example, aluminum content of thewell can be adjusted in other ways so that the energy gap is adjustedand the absolute value of the first gradient |ΔEI/ΔDI| can be smaller orgreater than |ΔEIII/ΔDIII|. For example, a material of the barrier caninclude aluminum nitride (AlN), a material of the well can includealuminum gallium nitride (Al_(x)Ga_((1-x))N; 0≤x≤1), and the introducedgas can include aluminum based gas. Since the energy gap of aluminumnitride is about 6.1 eV, which is greater than that of gallium nitride3.4 eV, in order to make the energy gap EI of the region I decreasealong the stacking direction and make the energy gap EIII of the regionIII increase along the stacking direction, aluminum content in theregion I can be decreased along the stacking direction and aluminumcontent in region III can be increased along the stacking direction.

As shown in FIGS. 3A to 3D, FIG. 3A shows flow rates as functions oftime while forming the well and the barrier in accordance with thesecond embodiment of the present application, FIG. 3B shows a diagram ofthe well and the barrier in accordance with the second embodiment of thepresent application, FIG. 3C shows the operational temperature as afunction of time while forming the well and the barrier in accordancewith the second embodiment of the present application, and FIG. 3D showsenergy bands and structures of the well and the barrier in accordancewith the second embodiment of the present application. The secondembodiment in FIG. 3A to FIG. 3D is similar to the first embodiment inFIG. 2A to FIG. 2D. One difference is in the structure of the well ofthe active layer. In the second embodiment, the active layer includes awell 601′ and at least two barriers 603′. Similarly, a region I 6010′ ofthe well 601′ is formed in the interval between t₁ and t₂ and a regionII 6012′ of the well 601′ is formed in the interval between t₃ and t₄.In the embodiment, a composition of the region I 6010′ is ranged fromGaN to In₀ ₂₅Ga_(0.75)N, a composition of the region II is In₀ ₂₅Ga₀₇₅N, and a composition of the region III 6014′ is ranged from In₀ ₂₅GaN₀₇₅N to GaN. Another difference between the first embodiment and thesecond embodiment is the interval between t₁ and t₂ shown in FIGS. 3A to3D is shorter than the interval between t₃ and t₄ and thus an indiumcontent difference per unit time in the interval between t₁ and t₂ isgreater than an indium content difference per unit time in the intervalbetween t₃ and t₄. Accordingly, as shown in FIG. 3D, an energy gap EI′difference of the region 6010′ per unit thickness DI′ is smaller than anenergy gap EIII′ difference of the region 6014′ per unit thickness DIII′from magnitude point of view. In other words, an absolute value of afirst gradient |ΔEI′/ΔDI′| in the second embodiment is greater than anabsolute value of a second gradient in the second embodiment|ΔEIII′/ΔDIII′|.

In the present application, the intervals between t₁ and t₂, t₂ and t₃,and t₃ and t₄ are not limited to 160 seconds, 60 seconds, and 60 secondsor 60 seconds, 60 seconds, and 160 seconds. In other embodiment, inorder to vary indium contents in different ways in different intervals,the intervals can correspond to different durations. For example, theinterval between t₁ and t₂ can be 2 to 3 times of the interval betweent₃ and t₄, the interval between t₁ and t₂ is shorter than the intervalbetween t₃ and t₄, or the interval between t₃ and t₄ and the intervalbetween t₂ and t₃ are longer than the interval between t₁ and t₂. Butthe present application is not limited hereto. As long as theoperational temperature difference (absolute value) per unit time in theinterval between t₁ and t₂ is different from the operational temperaturedifference (absolute value) per unit time in the interval between t₃ andt₄, absolute values of the indium contents per unit thickness (gradient)in the region I and region III are different from each other.Additionally, the first predetermined value, the second predeterminedvalue, and the third predetermined are not limited to 870 degreesCelsius, 755 degrees Celsius, and 875 degrees Celsius. The firstpredetermined value and the third predetermined value can be greaterthan the second predetermined value. In other embodiment, the firstpredetermined value and the third predetermined can be about 900 degreesCelsius and the second predetermined value can be smaller than 900degrees Celsius. Moreover, in other embodiment, the first predeterminedcan be between 870 degrees Celsius and 900 degrees Celsius, the secondpredetermined value can be between 750 degrees Celsius and 780 degreesCelsius, and the third predetermined value can be between 870 degreesCelsius and 900 degrees Celsius.

FIG. 4 shows energy bands of wells and barriers of light-emittingdevices in accordance with the first embodiment and the secondembodiment of the present application and the conventional art. In FIG.4, S represents the conventional light-emitting device, G represents thelight-emitting device of the first embodiment, and N represents thelight-emitting device of the second embodiment. In FIG. 4, materials ofa well S01 and a barrier S03 of the conventional light-emitting devicecan include indium gallium nitride (In_(0.25)Ga_(0.75)N) and galliumnitride GaN, respectively. The energy gap of the well S01 is fixed andis not varied with its thickness.

FIG. 5 shows internal quantum efficiency as functions of power for thelight-emitting devices in accordance with the first embodiment and thesecond embodiment of the present application and the conventional art.As shown in FIG. 5, S represents the conventional light-emitting devicein FIG. 4 and A represents both of the light-emitting devices of thefirst embodiment and the second embodiment. In FIG. 5, it shows theinternal quantum efficiency of the light-emitting devices of the firstembodiment and the second embodiment is higher than the internal quantumefficiency of the light-emitting device of the conventional art at thesame power value.

Please refer to FIGS. 6A and 6B. FIG. 6A shows output power as functionsof current density for the light-emitting devices in accordance with thefirst embodiment and the second embodiment of the present applicationand the conventional art. FIG. 6B shows the normalized efficiency asfunctions of current density for the light-emitting devices inaccordance with the first embodiment and the second embodiment of thepresent application and the conventional art. In FIGS. 6A and 6B, Srepresents the conventional light-emitting device, G represents thelight-emitting device of the first embodiment, and N represents thelight-emitting device of second embodiment. In FIG. 6A, thelight-emitting devices are measured at room temperature; in FIG. 6B, themeasured output power value of each of the light-emitting devices atroom temperature is normalized by the measured output power value ofeach of the light-emitting devices at low temperature so that a trend ofthe efficiency of the light-emitting device increasing with the currentdensity is investigated. As shown in FIG. 6A, at the same voltage and acurrent density of 69 A/cm² output power values of the light-emittingdevices of the first embodiment, the second embodiment, and theconventional art are 136.8 mW, 122.7 mW, and 110.1 mW, respectively. Theoutput power values of the light-emitting devices of the firstembodiment and the second embodiment are increased by 24.3% and 11.4%,respectively, compared with the conventional light-emitting device. Asshown in FIG. 6B, at a current density of 69 A/cm², the normalizedefficiency values of the light-emitting devices of the first embodimentand the conventional art are 73% and 61%, respectively. That means anefficiency declining rate of the light-emitting device of the presentapplication is lower than that of the conventional light-emitting deviceas the current density is increased.

In the embodiment, the external quantum efficiency (EQE) at a currentdensity of 13.8 A/cm² for the light-emitting devices of the conventionalart, the first embodiment, and the second embodiment are approximately59.6%, 68.3%, and 66.5%, respectively. The output power values of thelight-emitting device of the first embodiment and the second embodimentare increased by 11.7% and 5.8%, respectively, compared with thelight-emitting device of the conventional art. As above mentioned, at acurrent density of 13.8 A/cm² or 69 A/cm², the output power and theefficiency of the light-emitting devices of the first embodiment and thesecond embodiment are higher than that of the light-emitting device ofthe conventional art.

Please refer to FIGS. 7A, 8A and 9A. FIGS. 7A to 9A show simulations ofconcentrations of carriers versus position and energy bands of the welland the barrier versus position under bias. FIG. 7A shows concentrationof carriers as functions of position and energy bands as functions ofposition for the well and the barrier for the light-emitting device inaccordance with the conventional art. FIG. 8A shows concentration ofcarriers as functions of position and energy bands as functions ofposition for the well 601 and the barrier 603 in accordance with thefirst embodiment of the present application. FIG. 9A shows concentrationof carriers as functions of position and energy bands as functions ofposition for the well 601′ and the barrier 603′ in accordance with thesecond embodiment of the present application. In FIGS. 7A, 8A, and 9A,the structures (i.e. barrier, well, region I, region II, region III) arelabeled. As shown in FIGS. 7A, 8A, and 9A, higher concentration ofcarriers (electrons/holes) presents in the well. In the light-emittingdevice of the first embodiment or the second embodiment, a position of apeak value of the concentration of electrons s is closer to a positionof a peak value of the concentration of holes, compared with theconventional light-emitting device. It means that each wave functiondistribution of the electrons and the holes of the first embodiment andthe second embodiment overlaps more than that of the light-emittingdevice of the conventional art and the recombination rates in thelight-emitting devices of the first embodiment and the second embodimentare greater than the recombination rate of the conventionallight-emitting device. As shown in FIG. 8A, the variation of the energygap in region I 6010 of the well is smaller than the variation of theenergy gap in the region III 6014. While operating the light-emitting ofthe first embodiment, the electrons move from the region I 6010 to theregion III 6014 and the holes move from the region III 6014 to theregion I 6010. With the above structure, the speed of the holes isincreased and the movement of electrons is restrained so as to increasethe efficiency and decrease electrons overflow.

FIGS. 7B, 8B and 9B show simulations energy bands of the well and thebarrier and Femi energies of electrons and holes under bias. FIG. 7Bshows energy bands of the well and the barrier and Femi energies ofelectrons and holes for the light-emitting device in accordance with theconventional art. FIG. 8B shows energy bands of the well and the barrierand Femi energies of electrons and holes in accordance with the firstembodiment of the present application. FIG. 9B shows energy bands of thewell and the barrier and Femi energies of electrons and holes inaccordance with the second embodiment of the present application. Thereare four lines shown in the FIGS. 7B, 8B, and 9B. An upper line and anupper broken line in figures represent a conduction band profile andFemi energy of electrons, respectively; a lower line and a lower brokenline in figures represent a valance band profile and Femi energy ofholes, respectively. In comparison with FIG. 7B, the Femi energy ofelectrons in FIG. 8B is away from the minimum (valley value) of theconduction band. It means the probability that electrons present in thewell 601 is higher than the probability that electrons present in thewell S01. Additionally, the area (as slash lines shown) defined by theFemi energy and the conduction band in FIG. 8B is higher than that of inFIG. 7B. That represents the amount of electrons in the well 601 ishigher than that of in the well S01.

Please refer to FIG. 10 shows a simulation of the recombination rate asfunctions of position for the light-emitting devices in accordance withthe first embodiment of the present application and the conventionalart. S, G, and N represent a conventional light-emitting device, thelight-emitting devices of the first embodiment and the secondembodiment, respectively. As shown in FIG. 10, recombination rates ofthe active layers of the light-emitting devices of the first embodiment,the second embodiment are greater than that of the conventionallight-emitting device.

Please refer to FIG. 11, FIG. 11 shows a simulation of the normalizedefficiency as functions of current density for the light-emittingdevices in accordance with the first embodiment of the presentapplication and the conventional art. S and G represent a conventionallight-emitting device without a polarization field and thelight-emitting device of the first embodiment without a polarizationfield, respectively. S-P and G-P represent a conventional light-emittingdevice with a polarization filed (0.7Mvolt·cm⁻¹) and the light-emittingdevice of the first embodiment of the present application with apolarization filed (0.7Mvolt·cm⁻¹), respectively. As shown in FIG. 11,regardless of a polarization filed, a declining rate of the efficiencyof the light-emitting device of the first embodiment of the presentapplication is smaller than that of the conventional light-emittingdevice.

The principle and the efficiency of the present application illustratedby the embodiments above are not the limitation of the presentapplication. Any person having ordinary skill in the art can modify orchange the aforementioned embodiments. Therefore, the protection rangeof the rights in the present application will be listed as the followingclaims.

What is claimed is:
 1. A light-emitting device comprising: a firstconductivity semiconductor layer; a first barrier on the firstconductivity semiconductor layer; a well on the first barrier andcomprising: a first region having a first energy gap; and a secondregion having a second energy gap, wherein the first region is closer tothe first conductivity semiconductor layer than the second region is; asecond barrier on the well; and a second conductivity semiconductorlayer on the second barrier; wherein the first energy gap is decreasedalong a stacking direction of the light-emitting device and has a firstgradient, the second energy gap is increased along the stackingdirection and has a second gradient, and an absolute value of the firstgradient is smaller than an absolute value of the second gradient. 2.The light-emitting device of claim 1, wherein the first energy gapand/or the second energy gap are/is varied linearly or stepwise.
 3. Thelight-emitting device of claim 1, wherein the well further comprises athird region between the first region and the second region, and thethird region has a third energy gap which is a constant or devoid ofgradient variation.
 4. The light-emitting device of claim 3, wherein amaterial of the first barrier and the second barrier comprises galliumnitride, a material of the well comprises indium gallium nitride, and anenergy gap of the first barrier and an energy gap of the second barrierare greater than an energy gap of the well.
 5. The light-emitting deviceof claim 4, wherein an indium content of the first region is increasedalong the stacking direction, an indium content of the second region isdecreased along the stacking direction, and an indium content of thethird region is a constant.
 6. The light-emitting device of claim 4,wherein the indium content of the first region and/or the indium contentof the second region are/is varied linearly or stepwise.
 7. Thelight-emitting device of claim 3, wherein a material of the firstbarrier and the second barrier comprises aluminum nitride, a material ofthe well comprises aluminum gallium nitride, an aluminum content of thefirst region is decreased along the stacking direction, an aluminumcontent of the second region is increased along the stacking direction,and an aluminum content of the third region is substantially a constant.8. The light-emitting device of claim 1, further comprising a substrateunder the first conductivity semiconductor layer, and a buffer layerbetween the first conductivity semiconductor layer and the substrate. 9.The light-emitting device of claim 1, further comprising a strainreleasing stack, wherein the strain releasing stack is on the firstconductivity semiconductor layer and comprises a superlattice structure.10. The light-emitting device of claim 9, wherein the superlatticestructure comprises alternately stacking indium gallium nitride layersand gallium nitride layers.
 11. A light-emitting device comprising: afirst conductivity semiconductor layer; a first barrier on the firstconductivity semiconductor layer; a well on the first barrier andcomprising: a first region having a first energy gap; and a secondregion having a second energy gap, wherein the first region is closer tothe first conductivity semiconductor layer than the second region is; asecond barrier on the well; and a second conductivity semiconductorlayer on the second barrier; wherein the first energy gap is decreasedalong a stacking direction of the light-emitting device and has a firstgradient, the second energy gap is increased along the stackingdirection and has a second gradient, and an absolute value of the firstgradient is greater than an absolute value of the second gradient. 12.The light-emitting device of claim 11, wherein the first energy gap andthe second energy gap is varied linearly or stepwise.
 13. Thelight-emitting device of claim 11, wherein the well further comprises athird region between the first region and the second region, and thethird region has a third energy gap which is a constant or devoid ofgradient variation.
 14. The light-emitting device of claim 13, wherein amaterial of the first barrier and the second barrier comprises galliumnitride, a material of the well comprises indium gallium nitride, and anenergy gap of the first barrier and an energy gap of the second barrierare greater than an energy gap of the well.
 15. The light-emittingdevice of claim 14, wherein an indium content of the first region isincreased along the stacking direction, an indium content of the secondregion is decreased along the stacking direction, and an indium contentof the third region is a constant.
 16. The light-emitting device ofclaim 14, wherein the indium content of the first region and/or theindium content of the second region are/is varied linearly or stepwise.17. The light-emitting device of claim 13, wherein a material of thefirst barrier and the second barrier comprises aluminum nitride, amaterial of the well comprises aluminum gallium nitride, an aluminumcontent of the first region is decreased along the stacking direction,an aluminum content of the second region is increased along the stackingdirection, and an aluminum content of the third region is substantiallya constant.
 18. The light-emitting device of claim 11, furthercomprising a substrate under the first conductivity semiconductor layer,and a buffer layer between the first conductivity semiconductor layerand the substrate.
 19. The light-emitting device of claim 11, furthercomprising a strain releasing stack, wherein the strain releasing stackis on the first conductivity semiconductor layer and comprises asuperlattice structure.
 20. The light-emitting device of claim 19,wherein the superlattice structure comprises alternately stacking indiumgallium nitride layers and gallium nitride layers.